It is often desirable to weld or fuse together a multiplicity of sheets of thermoplastic materials, such as polyethylene, PVC or nylon, at a work site. The sheets are joined to produce a much larger sheet which can be used for such things as building covers, large tarpaulines or covers and for roofing material. It is more efficient to transport the bulk materials to the work site and join them there rather than joining them at a factory into a large, bulky, single sheet, then folding, transporting and unfolding that sheet.
Machines of this type are generally motor driven, self propelled machines in which a drive motor is connected through a drive train to one or more drive wheels. A heater, most commonly a hot air blower, and a feed means are provided on the machine for guiding the opposed overlapping edges of the sheets into the machine as the machine travels along the edges. The overlapping edges are guided past a heater or heat applying means through a compressing structure and then out of the machine.
In the operation of previous, conventional machines, the machine is initially set up, warmed and positioned on the sheets by an operator who then actuates the machine and guides it along the overlapping edges. Machines are manufactured and sold by companies such as Leister, Zinser and Pfaff. Other machines of this general type are shown in U.S. Pat. Nos. 4,050,972; 1,924,523; 2,556,476; 4,146,419; and others which have been cited in connection with the filing of this patent application.
Welding machines of this type require the application of heat to the overlapping sheets with the temperature range of approximately 400.degree. F.-600.degree. F. The most common prior art machines heat air and blow it upon the interfacing surfaces near the edges of the thermoplastic sheets. The hot air system, however, has several disadvantages. First, the air heating system must be heated to operating temperature prior to operation. This may take as long as fifteen minutes during which the energy used for heating the air is entirely wasted since the air is not sufficiently warm to raise the temperature of the sheet to the welding temperature during normal steady travel along the overlapping edges. Furthermore, during operation, the heat produced by the hot air heaters is not efficiently transferred to the thermoplastic sheet. Such machines therefore typically require heaters consuming electrical energy at a rate of 2500 to 4500 watts.
The high power requirement in turn results in a machine which requires operation at 240 volts and at a high current, typically in excess of 20 amps. The electrical cables and the machine itself which is necessary to operate at such a high power tends to be large, heavy and bulky and therefore difficult to transport to many work places, particularly to a building roof and the like. They are also difficult to move about and use during operation as a result of their size and bulk. There is therefore a need for a welding machine which consumes considerably less electrical energy and as a result may be lighter in weight and more conveniently transported and economically used. As a further convenience a machine is needed which can be energized from a standard 120 volt power source.
Another difficult with such prior art machines is that, after they are warmed up and positioned in place and their heat energy is directed to the plastic sheet by the operator, the operator has only a few seconds in which to begin the travelling motion of the welding machine. If he fails to move sufficiently fast, the machine will melt through the thermoplastic sheets forming a hole or other defect in the seam.
There is, therefore, a need for a machine which is not only more economical and efficient to use, but further, which is more convenient and can operate within less critical time tolerances requiring less operator skill and dexterity.
Although alternative heating sources might be utilized in a welding machine, one difficulty with them is that they do not instantaneously heat up to operating temperature or cool down to a nonoperating temperature at which they will not melt the plastic sheet. A finite time interval is required during which the temperature gradually increases or decreases. If such a heat source is positioned to transfer its heat to the overlapping edge of the sheet material when it is first turned on and is not moved until it reaches operating temperature, it will melt through the sheet material prior to reaching operating temperature and thus produce a defect in the seam. Furthermore, if at the instant the energy is applied to begin raising the temperature of the heat source the machine begins travelling at normal steady state linear velocity along the overlapping edges, suitable welding will not occur until the heat source reaches operating temperature. There will therefore be a considerable length along the overlapping edges which will not be joined properly.
There is, therefore, a need for a welding machine which can be positioned upon the plastic sheets with the sheets properly fed into the machine, then actuated and energized to begin moving and welding while its heater is heating up to operating temperature.
Previous welding machines require the continuous attention of the operator for steering them properly along the edges to be welded together. There is a need for a self-steering mechanism for use with such machines which can guide the welding machine along the overlapping edges without requiring steering by the operator. With no operator attention being required as the machine travels and welds, an operator's exposure to potentially dangerous fumes and gases is substantially reduced.